The present invention generally relates to defect detecting apparatuses for rotary recording mediums, and more particularly to a defect detecting apparatus for detecting defects such as a scratch, stain, dirt, deformed convex portion, deformed concave portion, or the like on a recording surface of a rotary recording medium which has information signals recorded on a spiral track as variations in geometrical configuration.
A rotary recording medium (hereinafter simply referred to as a disc) on which analog information signals such as a composite video signal and an audio signal are frequency-modulated and recorded on a spiral track or concentric tracks as variations in geometrical configuration such as rows of intermittent pits, is known. When manufacturing such a disc, a deformed convex portion which rises from the disc surface and a deformed concave portion which sinks from the disc surface, are sometimes formed on the disc surface during the manufacturing process. In addition, a stain, dust particle, dirt, or the like may adhere on the disc surface and cover the pits during the manufacturing process, and make the disc defective.
In a reproducing apparatus which reproduces pre-recorded information signals from the disc as variations in electrostatic capacitance between the disc and an electrode of a reproducing stylus which slides over the disc surface, the reproducing stylus may undergo an unwanted irregular movement when the disc has the defects described above. When the reproducing stylus slides over the deformed convex portion, for example, the reproducing stylus jumps upwardly at the deformed convex portion and bounces for a predetermined time before making smooth contact with the disc surface. While the reproducing stylus is separated from the disc surface, the pre-recorded information signals cannot be reproduced from the disc, and a dropout occurs in the reproduced signal. In addition, the reproducing stylus may jump in the track width direction at the deformed convex portion, and in this case, the reproducing stylus will scan over the wrong track and introduce a tracking error. A dropout similarly occurs in the reproduced signal when the reproducing stylus scans over the deformed concave portion, scratched portion, and portion with stain or dirt, and it is impossible to obtain a reproduced picture of a high quality in such cases.
Hence, the defects of the disc were conventionally detected in the disc manufacturing plant. According to a conventional method of detecting defects of the disc, the pre-recorded information signals (at least including the composite video signal) were picked up and reproduced by the reproducing stylus, and were then supplied to a monitoring receiver through a known reproducing circuit system. An inspector monitored the picture on the monitor receiver, and judged that a defect existed on the disc when a disturbance occurred in the picture which was being monitored.
However, according to the conventional defect detecting method, it was the limit for one inspector to simultaneously monitor two monitoring receivers. Further, it took approximately 1/2 the normal reproducing time of the disc to carry out the defect detection with respect to one disc. Moreover, because the defect detection was carried out by a person, it was possible for the person to miss a disturbance in the picture, and the standard for the defect detection differed according to the individual. As a result, there was a problem in that the reliability of the defect detection was poor due to the dependence on the inspector.
When a slightly deformed convex portion exists on the disc surface, such a slightly deformed convex portion may be scraped off as the reproducing stylus scans over this slightly deformed convex portion. In this case, the slightly deformed convex portion will no longer exist on the disc surface after it is scraped off by the reproducing stylus, and the defective disc will turn out to be an acceptable disc. However, such a defective disc was simply detected as being a defective disc according to the conventional defect detecting method, because the disc was played for the first time. In other words, since a dropout occurred in the reproduced signal when the reproducing stylus scanned over the slightly deformed convex portion while scraping off this slightly deformed convex portion, the disc was simply detected as being a defective disc even though the disc would have been detected as being acceptable if the disc was played for the second time.